1. Field of the Invention:
The present invention relates to a partial discharge detecting system of a gas-insulated switchgear that detects partial discharge generated in the interior of a gas-insulated switchgear used in a substation etc.
2. Discussion of the Background:
In recent years, what are known as gas-insulated switchgears have become common, wherein a disconnector or circuit breaker, etc. is accommodated in a sealed enclosure filled with SF.sub.6 gas, which has excellent insulating and arc-extinguishing properties. Such switchgears offer the advantages of small size and high performance.
A vital part of such a gas-insulated switchgear is the means used to detect abnormality occurring in the sealed enclosure. One such detecting means is a partial discharge detecting system that detects partial discharge occurring in the interior of the gas-insulated switchgear.
FIG. 1 shows an example of such a conventional partial discharge detecting system. A metal enclosure 1 of a gas-insulated switchgear 12 is grounded to earth and filled with the insulating gas SF.sub.6. The enclosure is electrically divided by a bell-shaped insulation spacer 2 and coupled in the longitudinal direction.
Metal enclosure 1 accommodates a disconnector or circuit breaker constituting the gas-insulated switchgear. FIG. 1 shows part of the region where a conductor 3 supported by insulation spacer 2 is accommodated. This conductor 3 connects the transformer with the disconnector or circuit breaker.
A ring-shaped electrode 4 for checking the voltage is arranged in insulation spacer 2. A floating capacitance C.sub.1 is therefore present between conductor 3 and electrode 4 and a floating capacitance C.sub.2 is present between metal enclosure 1 and electrode 4.
A partial discharge detector 7 is provided with a filter 8, connected to electrode 4 and metal enclosure 1 by means of leads 5 and 6, that inputs the signals obtained through leads 5 and 6 and extracts the frequency of the partial discharge, an amplifier circuit 9 that amplifies the output signal of this filter 8, and a peak detector circuit 10 that detects the peak value of the output signal of this amplifier circuit 9.
Now, in a condition in which voltage is applied to conductor 3, floating capacitances C.sub.1 and C.sub.2 constitute a voltage-dividing circuit, so a dividing voltage is generated at the two terminals of floating capacitance C.sub.2. A high-frequency component created by the partial discharge in metal enclosure 1 is superimposed on this dividing voltage.
The high-frequency component contained in the dividing voltage is therefore extracted by filter 8 of partial discharge detector 7, and its peak value is detected by peak detector circuit 10 through amplifier circuit 9. This detected peak value is output to a measurement device 11. One can thereby ascertain, using this measurement device 11, whether partial discharge has occurred within metal enclosure 1, based on the presence and magnitude of the input signal.
FIG. 2 shows the causes of partial discharge generated in metal enclosure 1 and the pattern of the resulting partial discharge pulse. Reason for partial discharge (1) may be further subdivided into case (1 - 1), in which discharge is produced due to the presence of bubbles or vacancies in the insulator in metal enclosure 1, and case (1-2), in which discharge is produced due to the formation of a gap at the surface of contact of the insulator and conductor.
In reason for partial discharge (2), discharge is produced when part of the insulator projects into the region in metal enclosure 1 that is filled with insulating gas (sharp edge). In reason for partial discharge (3), discharge is produced when part of the metal projects into the region in metal enclosure 1 that is filled with insulating gas (loose metal particles). In reason for partial discharge (4), discharge is produced when there is poor contact at locations where one conductor is in contact with another conductor in metal enclosure 1.
As can be seen from the figure, these reasons for partial discharge (1) to (4) result in discharge pulses having different patterns. The pattern of the discharge pulse is also different, depending on whether discharge has just started (left-hand column labeled "STARTING") or whether a stable condition of continuing discharge has been produced (right-hand column labeled "CONTINUING"). The sine wave waveform in the figure is the fundamental wave component, i.e., the 50 Hz mains frequency component of the voltage that is applied to conductor 3 while the small vertical lines at the zero axis represent the noise introduced into the sine wave waveform due to the partial discharge pulse. It can be seen that there are different phase differences between the discharge pulse and the fundamental wave component of the voltage, depending on the type of abnormality that gives rise to the partial discharge.
However, in the case of the conventional partial discharge detector, only the magnitude of the partial discharge pulse is detected and the phase difference with the fundamental wave component of the voltage is not detected, so the cause of the occurrence of partial discharge cannot be detected. Also, even the signal detected as the partial discharge pulse may consist of random noise superimposed on the fundamental wave of the voltage, so in determining whether partial discharge is taking place or not, a large range of error has to be taken into account. This therefore gives rise to a problem of loss of measurement accuracy.
A technique which has recently been employed is to convert the detected partial discharge pulse into an optical signal, which is then transmitted to a monitoring room. However, in the conventional partial discharge detector, this point was not taken into consideration.